The present disclosure relates to compressors used for compressing a fluid such as air, and more particularly relates to compressors and compressor housings including injection passages for re-circulating a portion of a compressor fluid.
Compressors, such as axial and centrifugal compressors, are used in a variety of applications for compressing fluids. Centrifugal compressors are particularly suitable for applications in which a relatively low overall pressure ratio is needed. A single-stage centrifugal compressor can achieve peak pressure ratios approaching about 4.0 and is much more compact in size than an axial flow compressor of equivalent pressure ratio. Accordingly, centrifugal compressors are commonly used in turbochargers for boosting the performance of gasoline and diesel engines for vehicles.
In various applications, it is important for the compressor to have a wide operating envelope, as measured between the “choke line” at which the mass flow rate through the compressor reaches a maximum possible value because of sonic flow conditions in the compressor blade passages, and the “surge line” at which the compressor begins to surge with reduction in flow at constant pressure ratio or increase in pressure ratio at constant flow. Compressor surge is a compression system instability associated with flow oscillations through the whole compressor system. It is usually initiated by aerodynamic stall or flow separation in one or more of the compressor components as a result of exceeding the limiting flow incidence angle to the compressor blades or exceeding the limiting flow passage loading. For example, in a turbocharger, compressor surge can occur when the engine is operating at high load or torque and low engine speed, or when the engine is operating at a low engine speed with a high rate of exhaust fluid recirculation from the engine exhaust side to the intake side. Compressor surge can also occur when a relatively high specific torque output is required of an engine with a variable nozzle turbine (VNT) or an electrically assisted turbocharger. Additionally, surge can occur when a quick boosting response is required using an electrically assisted turbocharger and/or VNT turbocharger, or when the engine is suddenly decelerated, e.g., if the throttle valve is closed while shifting between gears.
As a result of any of the foregoing operating conditions, the compressor can surge as the axial component of absolute flow velocity entering the compressor is low in comparison to the blade tip speed in the tangential direction, thus resulting in the blades of the compressor operating at a high incidence angle, which leads to flow separation and/or stalling of the blades. Compressor surge can cause severe aerodynamic fluctuation in the compressor, increase the noise of the compressor, and reduce the efficiency of the compressor. In some cases, compressor surge can result in damage to the engine or its intake pipe system.
Thus, there exists a need for an improved apparatus and method for providing compressed fluid, such as in a turbocharger, while reducing the occurrence of compressor surge. In some cases, the prevention of compressor surge can expand the useful operating range of the compressor.